One of the “Holy Grails” for antenna engineers working in the aircraft and other vehicle fields, where aerodynamic drag and vehicle profile are important, is achieving an antenna with wide bandwidth, high efficiency, a convenient radiation pattern, and a small addition to vehicle profile. This latter characteristic may be considered, with the use of other words, as a need for a vehicle “conformal antenna.” The need for these characteristics extends significantly into the world of the stealth aircraft since non-conformal protuberances on an airframe provide a substantial radar signal reflection or return point addition to the aircraft's radar signature. These desired antenna characteristics typically are, however, conflicting in nature and thus an antenna engineer must often make trade-offs amongst these needs.
There are a variety of conformal antennas used in microwave signal and aircraft practice, but perhaps the most studied of these is the microstrip patch antenna. As a result of its relative simplicity with respect to both modeling and construction, the patch antenna has been a subject of extensive research and use for over thirty years. Munson [1] performed seminal work on this antenna as did Carver and Mink [2]. (Numbers of this configuration herein refer to entries in the list of references at the close of this specification.) The simple approximate models developed by these authors have been used since their publication and are now included in the subject matter of many Engineering School undergraduate antenna courses [3].
One of the major challenges associated with the patch antenna is however, the relatively narrow bandwidth such an antenna achieves [4]. Such antennas, if probe or microstrip transmission line fed, have a bandwidth of typically less than 5% and often less than 2% [3]. Increasing the substrate thickness used with these antennas can increase this bandwidth; however, surface waves can be excited in such patch antennas and this leads to a rather serious reduction in efficiency. This reduction can be limited by the introduction of shorting pins, or a cavity that have the effect of squelching surface waves. However, care must be used in achieving such surface wave reductions since placement of metal near the radiating edges of a patch antenna has a significant impact on its properties. Moreover since patch antennas are usually used in large arrays, in part because of their low cost and low gain, shorting pins or cavities cannot always be used due to the proximity of the antenna elements to each other. The result is strong surface wave coupling between adjacent antennas and this complicates the antenna synthesis task. Alternative feeding mechanisms can be used to increase the achieved bandwidth, without exciting surface waves; however, the achievable bandwidth is typically on the order of 20% to 60% [5] but certainly bandwidths of 2:1 or 10:1 are not achievable with any manner of feeding a patch antenna.
Another approach to increasing patch antenna bandwidth, without a commensurate reduction in efficiency involves the use of magneto-dielectric materials [6] in the antenna. However, the relatively high efficiency that can otherwise be achieved with patch antennas requires low loss magnetic materials. Such materials are difficult to realize at high frequencies, at frequencies greater than 1 gigahertz for example.
From another perspective, there are a group of antennas that are inherently of wide bandwidth and have reasonable efficiency. These antennas include printed spirals (including slot spirals), circular log-periodic arrays as well as helix, bicone, and sleeve antennas. A general theory concerning these and other frequency independent antennas has in fact been presented by Rumsey and is described by Thiele [3]. The first two of these wide band antennas are amenable to conformal installation as in an airframe while the latter types typically are protruding antennas. However, like the patch antenna, the radiation pattern for these antennas depends on feed conditions or mode of operation chosen and has a peak normal to the platform in which it is installed. Examples of feed conditions that will result in a pattern peak away from this direction include higher-order mode excitation for the patch or a phase array of elements with the excitation feed phases chosen to steer the beam. However, it is a well-known fact that for a finite array of elements, there are scan limits on the beam for such elements.
The antenna of the present invention provides what is believed to be a useful addition, perhaps even a breath of fresh air, to this antenna selection scene.